Various records of medical examination and consultation are daily produced in large volumes in medical organizations. Especially, an increase in the volume of medical pictures due to development and diffusion of image diagnostic equipment is posing a problem about how to preserve such medical pictures. Under such circumstances, the Ministry of Health and Welfare issued a circular notice to the effect that preservation of x-ray photograph, CT pictures, etc. may be made in the form of electronic media such as optical disc, magnetic disc, etc. in place of preservation of the original pictures.
With such changes in the medical environments, it is becoming necessary to accurately measure variable-density pictures of large surface area such as light transmission type X-ray photograph of chest for medical use, etc. up to the limit of visibility of human eyes and then decompose its density in pixels to store and reproduce it in numerical values.
By the way, methods and apparatuses for detecting the density of variable-density pictures currently known generally include the following:
The first method and apparatus is one which is designed to use, as light source, a uniform-plane light source "a" as shown in FIG. 5, condense the light from this light source "a" by a convex lens "b" after transmission through a film "f", and then detect the density by means of a plane CCD sensor "m". In FIG. 5, dotted lines indicate optical paths. The same is true with FIG. 6 to FIG. 10 that follow.
The second method and apparatus is one which is designed to use, as light source, a uniform-plane light source "a" as shown in FIG. 6, condense the light from this light source "a" by a cylindrical lens "c" after transmission through a film "f", and detect the density by means of a line CCD sensor "n".
The third method and apparatus is one which is designed to use, as light source, a line-scan light source "d" as shown in FIG. 7, condense the light from this light source "d" by a glass fiber bundle "g" after transmission through a film "f", and detect the density by means of a PIN photo diode "p". In FIG. 7, the arrow mark indicates the direction of scanning. The same is true with FIG. 8 to FIG. 10 that follow.
The fourth method and apparatus is one which is designed to use, as light source, a line-scan light source "d" as shown in FIG. 8, condense the light from this light source "d" on an integrating sphere "i" after transmission through a film "f", and detect the density by means of a PIN photo diode "p".
The fifth method and apparatus is one which is designed to use, as light source, a line-scan light source "d" as shown in FIG. 9, condense the light from this light source "d" by a glass fiber bundle "g" after transmission through a film "f", and detect the density by means of a PMT (photomultiplier tube) "t".
The sixth method and apparatus is one which is designed to use, as light source, a line-scan light source "d" as shown in FIG. 10, condense the light from this light source "d" on an integrating sphere "i" after transmission through a film "f", and detect the density by means of a PMT (photomultiplier tube) "t".
In addition to the above, there are also those which use LED or EL as light source.
However, said respective methods and apparatuses have the following problems:
For example, for digitizing an X-ray photograph of chest, a picture element resolution (positional resolution) of 4444.times.5398 pixels is required, but no plane CCD sensor "m" used for such a large surface area is put into practical use at present because of a variety of problems. For that reason, the first method and apparatus causes a problem not to detect density on the object X-ray photograph of chest, etc.
Moreover, while, as line CCD sensor "n", those of 1024, 2048 and 4096 pixels are developed currently, their resolution is limited to approximately 256 (2.sup.8) or so at best and, therefore, they have a problem of being unfit for use as detecting element for X-ray photograph of chest, etc. which is said to require a resolution of 10000 or so. For that reason, the second method and apparatus causes a problem not to detect of density on the object X-ray photograph of chest, etc. as the first one.
Furthermore, in the case where the light is condensed with the use of a glass fiber bundle "g", there is a possibility of detecting the density on the object X-ray photograph of chest, etc. by using the smallest glass fiber of 50 .mu.m in diameter currently developed, because the positional resolution of the glass fiber bundle "g" depends on the glass fiber diameter. However, the operation of arranging glass fibers of a diameter of 50 .mu.m at equal intervals on a row cannot be easily mechanized and therefore must be carried out by hand work, causing a problem of low productivity and high cost.
Still more, in the case of detection made by using an integrating sphere "i", it is necessary to provide a large number of integrating spheres "i" in parallel to improve the positional resolution because the positional resolution is determined by the size of focal point of the light source, causing a problem of large equipment size and, inevitably, of high equipment cost. An integrating sphere "i" is realized, as shown in FIG. 11, by providing an entrance window and a light detecting element such as photomultiplier tube, etc. on a sphere in such a way as to catch the incident light the entrance window by integrating it in the sphere and detect the quantity of light caught with the light detecting element.